Data migration service FAQ

Below are common questions about running a migration with the data migration service.

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Before you migrate data

How do I ensure that email messages are RFC-compliant? 

Messages must be RFC-compliant to be migrated with the data migration service. If they aren't, you’ll see “Our system has detected that this message is not RFC 2822 compliant" or the message might be migrated with the wrong date

Here are some common reasons why messages aren't RFC-compliant:

  • The From field can’t be empty and must only have a single address.
  • The domain in the From field can't be malformed.
  • The Date field must have 4 digits for the year, not 2.
  • The time of day field must be in the range 00:00:00 through 23:59:60.

If you can edit the message in the source mailbox, you might be able to correct the message headers and remigrate. If not, you should migrate the messages some other way (for example, by forwarding them to the new account using your email client). For information about correcting the messages, contact your legacy account provider.

Can resellers migrate data for a customer? 

No, resellers can't migrate data using the Reseller console. Migrations must be performed using the Google Admin console.

Can I migrate IMAP email folders that are shared or public?

The data migration service does not support migrating shared or public IMAP folders.

Running a migration

How long will a migration take?

The duration depends on a number of factors, such as:

  • The number of users you’re migrating.
  • The migration start date you select—the longer the timeframe, the longer a migration takes.
  • The number and size of messages being migrated.
  • The performance of the legacy server you’re migrating from.
  • Network bandwidth and latency between Google and the legacy server.
Can I migrate all email messages in one migration?

Yes, you can choose a custom date range to specify the entire period on your legacy account. If you have many years or multiple gigabytes of data, we don't recommend migrating all email messages in a single migration. It takes a long time to complete a migration of this size. For details on custom date ranges, see Specify a migration start date

What happens to email received in the legacy account when a migration is running?

If you’re migrating from Microsoft® Exchange® or Office 365®, any new messages delivered into the legacy account during the migration are transferred until you exit the migration. If you’re migrating from any other platform, any new messages delivered to the legacy account during the migration are not migrated. 

Can I migrate from multiple sources to one user account?

No. The data migration service is designed to allow only one migration at a time to a specific user.

We don't recommend having multiple concurrent API requests migrating data into single account.

During a migration

Why does my migration look like it's stuck at 99%?

You’ll see 99% when all email is migrated. After everything is migrated, the data migration service applies any labels to the migrated email, which can take time. When the labels are applied, you should see that the migration is complete (100%).

You might also see this issue if the estimated number of emails to migrate exceeds the actual number of messages. The migration will report 99% until the migration completes. This process might take some time.

Rerunning a migration

If I rerun a migration, will email messages be duplicated?

No. If you exit and rerun a migration, any messages that are already migrated won't be duplicated. 

The data migration service checks if each message is already in the target mailbox before trying to migrate it. It skips messages that are already there, instead of downloading them again from the source mailbox.

If I rerun a migration, will calendar events or secondary calendars be duplicated?

No. If you exit and rerun a migration, any calendar events or secondary calendars that are already migrated won't be duplicated.

However, if a user changes the name of a secondary calendar between migrations, that secondary calendar will be duplicated when it's remigrated.

After a migration

Why do I see "Migration/DMS-Temp" labels in the new Gmail account?

If you’re migrating from Gmail, users who sign in to their email accounts might temporarily see "Migration/DMS-Temp." This label is deleted when the migration is complete.

In Gmail, messages appear with the wrong date

Migrated messages might show the migration date and time, rather than the time and date of the original message.

This most likely occurred because the date header of the original message wasn't compliant with RFC 5322. If a message has a date header that isn't formatted correctly, Gmail applies the migration time and date to the message.

Why is there a difference between the total amount of messages following a migration?

Following a migration, you might notice that the number of email messages in the source environment exceeds the number of messages migrated to Gmail. 

This discrepancy might be due to the difference between folders in Exchange and IMAP mail servers and labels in Gmail. In Exchange and on IMAP mail servers, when you assign a message to more than one folder, the message is duplicated. Following a migration, the duplications are removed in Gmail and different labels are applied instead. 

To verify that all messages have been migrated, check the logs for any errors. For details, go to Monitor a migration.

If you think you’re missing some messages, check whether: 

  • The message, including attachments, is not bigger than 25 MB. You can’t migrate messages bigger than 25 MB. For details, see Send attachments with your Gmail message.
  • An attachment to the message is not blocked by Gmail. Gmail blocks certain types of attachments, such as executable files. For details, see File types blocked in Gmail.
  • The message is in a folder or within a date range that’s part of the migration.


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